FeC r2 O4 spinel to near megabar pressures: Orbital moment collapse and site-inversion facilitated spin crossover

The interplay between lattice, orbital, and spin degrees of freedom in iron chromite (spinel [Fe]{Cr2}O4) has been investigated to near megabar pressures. The cubic-to-tetragonal transition, from static Jahn-Teller distortions at the Fe locality, rises from TJ-T∼135K at ambient pressure to 300 K by ∼12GPa. The tetragonal distortion progressively increases and orbital moment quenching is triggered beyond ∼24GPa, as monitored by the magnetic hyperfine field Hhf at Fe sites. In the range 30-60 GPa, original Fe2+ tetrahedral sites with an unquenched orbital moment (Hhf∼20T) coexist with newly evolved Fe2+ at tetrahedral sites having the orbital moment quenched and a resultant large Hhf∼35T. Additionally, new Fe2+ sites having distinguishable orbital moment quenching signatures (Hhf∼42T) are discerned. Those sites also have other Fe nuclear hyperfine interaction parameter values typical of Fe2+ in octahedral coordination. There is a concurrent change to a steeper decrease of unit-cell volume as pressure rises above ∼30GPa. These electronic and lattice responses are interpreted as signatures of progressive partial spinel inversion from high-spin Fe↔Cr tetrahedral/octahedral site exchange, triggered near ∼30GPa. Beyond 60 GPa a new diamagnetic low-spin Mössbauer spectral component emerges. This is preceded by an inflection and discontinuity in the pressure dependence of the resistance and tetragonal unit-cell volume, respectively. By ∼93GPa, half of the iron is low spin in octahedral sites from evolved tetragonal-spinel inversion processes. The remainder reside in tetrahedral high-spin sites with Hhf∼30T. The charge gap from electron correlations does not close, despite an anticipated appreciable band broadening from a ∼30% unit-cell volume reduction upon pressurization to ∼93GPa. This is attributable to an increase in effective on-site repulsion (Hubbard Ueff) with increasing pressure or at spin crossover, specific to the Cr3+(d3) and Fe2+(d6) electronic configurations, respectively. Thus a ∼200meV "Mott" gap prevails in the mixed spin-state of the partially inverted tetragonal phase [Fe1-xCrx]{FexCr2-x}O4(x∼0.5) up to near a megabar (100 GPa) densification.